(a) Field of the Invention
The present invention relates to lithium secondary batteries. More particularly, the present invention relates to active material for a positive electrode used in lithium second batteries and a method of manufacturing the same in which structural and thermal stability of the active material are improved, thereby greatly enhancing the overall safety of the battery.
(b) Description of the Related Art
With the proliferation in the use of portable electronic devices in recent times, coupled with advancements made enabling increasingly smaller sizes and weights for these devices, research is being actively pursued to improve energy density capabilities of lithium secondary batteries.
Lithium secondary batteries utilize material that is able to undergo lithium ion intercalation and deintercalation respectively for a negative electrode and a positive electrode, and are filled with organic electrolyte or polymer electrolyte, which enable movement of lithium ions inside the battery (i.e., back to the negative electrode in the form of an ionic current). The lithium secondary battery generates electrical energy by processes of oxidation and reduction which take place when lithium ions undergo intercalation and deintercalation in the negative electrode and the positive electrode, respectively.
In the past, although lithium metal was used as the negative electrode active material in lithium secondary batteries, a serious problem of dendrite forming on a surface of the lithium metal resulted during charging and discharging. This may cause a short circuit, or more seriously may lead to the explosion of the battery. To prevent such problems, carbonaceous material is now widely used for the negative active material. Carbonaceous material is able to alternatingly either receive or supply lithium ions while maintaining its structural integrity and electrical properties, and half of a potential of the cell is identical to that of lithium metal during insertion and separation of ions.
For the active material of the positive electrode in secondary batteries, a metal chalcogenide compound, enabling insertion and separation of lithium ions, is generally used, i.e. composite metal oxides such as LiCoO2, LiMn2O4, LiNiO2, LiNi1xe2x88x92xCoxO2 (0 less than X less than 1), and LiMnO2. Regarding the advantages and disadvantages of these different materials: the Mn-based active materials, LiMn2O4, and LiMnO2, can easily synthesize, are less expensive than the other materials and give minimal negative affects on the environment, but capacities of these materials are low; LiCoO2 is widely used as it exhibits an electrical conductivity of roughly 10xe2x88x922 to 1 S/cm at room temperature, provides a high level of battery voltage, and has exceptional electrode characteristics, but is unsafe when charging or discharging at a high rate, and is more costly than the other materials; and LiNiO2 has a high discharge and charge capacity and is the least expensive of the above active materials for the positive electrode, but does not synthesize easily.
Generally, such composite metal oxides are manufactured by mixing with a solid raw material powder, and this mixture undergoes a solid phase reaction for providing plasticity to the mixture. For example, Japanese Laid-open Publication No. Heisei 8-153513 (Sony Corp.) discloses a method for manufacturing LiNi1xe2x88x92xCoxO2 (0 less than X less than 1) in which after a hydroxide containing Ni(OH)2 and Co(OH)2 or Ni and Co is mixed and heat treated, the hydroxide is ground and fractionated to diameter sizes of the particles. In another method, LiOH, Ni oxide and Co oxide are reacted, and after undergoing a first sintering at 400 to 580xc2x0 C. to form an oxide, a second sintering is performed at 600 to 780xc2x0 C. to manufacture a perfect crystalline active material.
However, in such conventional methods, the resulting active material has a low degree of both structural and thermal stability, reducing the safety of the battery.
The present invention has been made in an effort to fulfill the above need.
It is an object of the present invention to provide active material for a positive electrode used in lithium secondary batteries in which the active material has a high degree of structural and thermal stability.
It is another object of the present invention to provide a method of manufacturing the active material having the above characteristics.
To achieve the above objects, the present invention provides active material for a positive electrode used in lithium secondary batteries of Formula 1 below in which crystalline powder or semi-crystalline powder of Formula 1 is manufactured, and after coating the crystalline powder or semi-crystalline powder with metal alkoxide sol, the coated powder is heated, thereby producing an active material that is coated with a metal oxide on its surface.
LiA1xe2x88x92xxe2x88x92yBxCyO2xe2x80x83xe2x80x83(Formula 1)
where 0 less than xxe2x89xa60.3, and 0xe2x89xa6yxe2x89xa60.01.
In the Formula 1 above, A is an element selected from the group consisting of Ni, Co and Mn; B is an element selected from the group consisting of Ni, Co, Mn, B, Mg, Ca, Sr, Ba, Ti, V, Cr, Fe, Cu and Al; and C is an element selected from the group consisting of Ni, Co, Mn, B, Mg, Ca, Sr, Ba, Ti, V, Cr, Fe, Cu and Al.